Super austenitic stainless steels are used in the area of environment related equipment such as FGD equipment, chemical plants, and offshore equipment. The σ phase can easily precipitate in super austenitic stainless steels when compared with grade 304, due to its high chromium and molybdenum content. Nevertheless, the degree of σ phase precipitation in super austenitic stainless steel is better than SDSS.
3. High nickel alloys
For environment preservation or cost reduction, the application in chemical plants and the equipment used under elevated temperatures become severe and the range of use for high nickel alloys becomes increasingly greater. For welded pipes and tubes made of high nickel alloys, prevention of hot cracking and oxidized scale is important.
3.1. Prevention of hot cracking and blowholes
High nickel alloys are more likely to incur hot cracking and blowholes than grade 304, therefore welding current control, interpass temperature control, the selection of the applicable welding material, and the conditions of use and the cleanliness of the groove are important.
3.2. Prevention of oxidized scale
The oxidized scale after welding reduces corrosion resistance, so oxidized scale formation can be prevented by suitable gas shielding. In cases where oxidized scale forms after welding, this should be removed by pickling, grinding or sandblasting.
4. Square pipes
Some applications for the use of square pipes require a tight tolerance on the internal radius at the four corners. Grade 304 is temperature dependent due to its mechanical properties, so NAS TOA preheats material in winter to ensure a tight tolerance.
5. Some applications
5.1. High-performance alloys
Welded pipes and tubes made of high-performance alloys such as IncoloyTM, InconelTM, HastelloyTM, MonellTM and pure nickel are used for chemical plants, heat exchangers, and equipment used under elevated temperatures.
5.2. Special alloys
SDSS (UNS S32750) welded pipes and tubes are used for seawater desalination plants, FPSOs (Floating Production, Storage and Offloading), etc. 6Mo grade super austenitic stainless steel welded pipes and tubes are used for environmental related facilities and NAS354NTM (UNS N08354, ASME Code Case 2543, 23Cr-35Ni-7.5Mo-0.2N) with a corrosion resistance level between 6Mo grade super austenitic stainless steel and HastelloyTM is used for waste disposal facilities and salt production facilities.
5.3. Special specification
Pipes and tubes used in a nation’s infrastructure, such as electric power plants and gas facilities are required to meet strict specifications, even when the material may be a common grade such as 304, 304L, 316, 316L. NAS TOA also supplies pipes for the Large-scale Cryogenic Gravitational Wave Telescope (LCGT) as a national level project in Japan.